Pacific bluefin tuna carried radioactivity from Japan’s 2011 Fukushima Daiichi nuclear disaster all the way across the ocean to the shores of California, scientists reported Monday.

They didn’t bring much — the levels were far lower than, for instance, levels of naturally occurring potassium 40 that have existed in the ocean for centuries — but the radioactivity was enough to survive the fishes’ migration east to North America from the Western Pacific, which they undertake when they’re around a year old, said doctoral student Daniel Madigan, who studies the migration patterns of tuna at Stanford University.

Last year's March 11 quake and tsunami set off meltdowns at the Fukushima Daiichi nuclear power plant.

“We showed that a bluefin tuna is capable of picking up radioactive material and transporting it across the ocean. That’s new. Traditionally people don’t think of migratory animals as transport vectors for radioactive materials,” he said.

Madigan made the discovery in samples of fish he collected during the summer of 2011, about five months after the disaster, when “Fukushima was on everyone’s mind.” Not sure what he’d find, he collected bits of Pacific bluefin tuna flesh from the catches of fishermen in San Diego and sent 15 samples from smaller fish (which, being younger, would have been the most recent migrants from Japan) to Nicholas Fisher’s laboratory at Stony Brook Universityin New York. There they were analyzed for the presence of radioactivity from Fukushima.

Madigan said neither he nor Fisher thought they’d see much. They assumed the radioactivity would have been diluted as the fish got away from the coast. “We thought it was unlikely they’d pick up enough of a signal and hang onto it long enough to reach California,” he said.

But upon analysis, the researchers found signals from Fukushima—isotopes called Cesium-134 and Cesium-137 — in all 15 samples they tested. When the team tested for the isotopes in bluefin tuna that migrated to California before the disaster and yellowfin tuna that are native to California waters, the radioactivity wasn’t present, which indicated that it came from Fukushima, Fisher said.

The amount of Cesium 134 and 137 detected in the fish “didn’t come close to exceeding safety limits,” Madigan said, noting that what was in the fish, per gram, is lower than the amount of naturally occurring radioactive potassium found per gram in a banana.

He said he hopes to measure radioactive cesium levels in bluefin tuna again this year, looking at a wider range of bluefin sizes, as well as radiation in other species. The measurements could help researchers study migration patterns in the animals, he added.